1. Field of the Invention
The present invention relates to an optical sheet and a manufacturing method thereof, a backlight, and a liquid crystal display, and in particular to an optical sheet without a sidelobe and having a wide field of view and a manufacturing method thereof, and a backlight and a liquid crystal display using the optical sheet.
2. Description of the Related Art
A display or TV monitor for a PC has changed from a conventional CRT to a planar display satisfying requirements of miniaturization and thinness, and especially, a liquid crystal display is one of the most widely used systems because of its long history in the market.
Because this liquid crystal display is not of a self-luminous type like an EL or plasma display, it includes a backlight composed of a light emitter on the back side thereof to enhance visibility. For this backlight, a type of a backlight called “edge light system” in which a light source such as a cold-cathode tube is disposed on one side or both sides thereof is said to be advantageous from the viewpoints of thinness, lightweight and electrical power saving, and has been widely used.
FIG. 1 shows a configuration of the backlight of the edge light system in a cross-section view. This backlight includes a light source 11 which is composed of a plurality of cold-cathode tubes 10 and a so-called “pseudo flat surface light source” in which light emitted from each of the plurality of cold-cathode tubes 10 is configured to be emitted from a surface on one side of the light source 11.
The light emitted from the cold-cathode tube 10 in the light source 11 is dispersed by a diffusing plate 12 and a diffusing sheet 13, and enters an optical sheet 14 which is a prism sheet having a function as a collection sheet. The incident light is refracted by an optical film, emitted with and the twisted angle irradiated to a liquid crystal panel not shown from behind.
However, the optical sheet 14, which is the prism sheet, has a triangular, prismatic and concavo-convex pattern formed continuously thereon, and accordingly, in the light which is emitted from the light source 11 and exits through the diffusing plate 12, the diffusing sheet 13 and the optical sheet 14, the sidelobe is produced to cause a problem of a narrow field of view of a panel, and light interference is generated to raise a problem of nonuniformity in luminance, presenting insufficient visibility of the panel. In order to solve these problems, Japanese Patent Application Laid-Open Nos. H 8-190806, H 7-294709, H 7-230002 and H 5-313004 have proposed that a shape of the triangular, prismatic and concavo-convex pattern of the optical sheet 14 which is the prism sheet is modified.